The present invention relates generally to liquid crystal display devices, and more particularly to a digital to analog converter for providing a voltage for driving a display screen of a liquid crystal display device pixels accurately shaded in correspondence with a digital or analog video signal.
Liquid crystal displays (LCDs) are commonly used in devices such as portable televisions, portable computers, control displays, and cellular phones to display information to a user. LCDs act in effect as a light valve, i.e., they allow transmission of light in one state, block the transmission of light in a second state, and some include several intermediate stages for partial transmission. When used as a high resolution information display, as in one embodiment of the present application, LCDs are typically arranged in a matrix configuration with independently controlled pixels. Each individual pixel is signaled to selectively transmit or block light from a backlight (transmission mode), from a reflector (reflective mode), or from a combination of the two (transflective mode).
An LCD pixel can control the transference for different wavelengths of light. For example, an LCD can have pixels that control the amount of transmission of red, green, and blue light independently. In some LCDs, voltages are applied to different portions of a pixel to control light passing through several portions of dyed glass. In other LCDs, different colors are projected onto the pixel sequentially in time. If the voltage is also changed sequentially in time, different intensities of different colors of light result. By quickly changing the wavelength of light to which the pixel is exposed an observer will see the combination of colors rather than sequential discrete colors. Several monochrome LCDs can also result in a color display. For example, a monochrome red LCD can project its image onto a screen. If a monochrome green and monochrome blue LCD are projected in alignment with the red, the combination will be full color.
The monochrome resolution of an LCD can be defined by the number of different levels of light transmission that each pixel can perform in response to a control signal. A second level is different from a first level when the user can tell the difference between the two. An LCD with greater monochrome resolution will look clearer to the user.
LCDs are actuated pixel-by-pixel, either one at a time or several simultaneously. A voltage is applied to each pixel and the liquid crystal responds to the voltage by transmitting a corresponding amount of light. In some LCDs an increase in the actuation voltage decreases transmission, while in others it increases transmission. When multiple colors are involved for each pixel, multiple voltages are applied to the pixel at different positions or times depending upon the LCD. Each voltage controls the transmission of a particular color. For example, one pixel can be actuated to allow only blue light to be transmitted while another allows only green. A greater number of different light levels available for each color results in a much greater number of possible color combination.
Converting a complex digital signal that represents an image or video into voltages to be applied to the pixels of an LCD involves circuitry that can limit the monochrome resolution. The signals necessary to drive a single color of an LCD are both digital and analog. It is digital in that each pixel requires a separate selection signal, but it is analog in that an actual voltage is applied to the pixel to determine light transmission. The conversion from a bit-representation of the desired light transmission, as communicated in the image or video signal, to an actual voltage that controls the light transmission can introduce errors that reduce the monochrome resolution of the LCD. For example, if a Digital-to-Analog Converter (DAC) takes as an input a bit-representation of voltage that includes 1024 voltage levels and outputs voltages between 0 and 16 volts, the ideal output levels would differ by 16 millivolts (mV). However, the electro-optical curve (EO curve) has most of the gray shade changes occur in voltages about 1V to 2V on either side of the center voltage of the display. In other words, a linear increase of the voltage provided by the DAC does not necessarily reflect the same increase in gray shade. In the specific region mentioned above, the sensitivity preferably should be about 4 times higher, in the above mentioned example, about 4 mV.
The embodiments of the present application are directed to a system and method for providing a control voltage for driving a liquid crystal display.
In one embodiment of the present application, a matrix of liquid crystal pixels is provided. A digital-to-analog (DAC) converter is coupled to the matrix and produces an output voltage that can be applied to one or more pixels in the matrix. The DAC receives multi-bit digital input and generates an output voltage according to its conversion function corresponding to the digital input. The conversion function of the DAC is specifically adapted to provide an optimum resolution within a voltage range in which most of the gray changes on the LCD occur.
In another embodiment the DAC comprises a non-linear conversion function which, for example, can be divided into a plurality of linear sub-ranges. The transition from one sub-range to an adjacent sub-range is non-linear.
The DAC, as shown in one or more embodiments, can consist of a plurality of standard DACs and a control logic which switches between the DACs to provide the different sub-ranges. Two or more DAC, can also be combined according to another embodiment of the present application.
In yet another embodiment the DAC comprises a comparator and an inverter coupled with a digital signal. Furthermore, the first switching circuit is controlled by the comparator for selecting the digital signal or the inverted digital signal are provided. An adder for adding a constant coupled with the switching circuit and a first digital-to-analog converter being coupled with the adder are also provided. For generating a second resolution a second digital-to-analog converter is coupled with the first switching circuit and second switching circuit are controlled by the adder for selecting one of the digital-to-analog converters. An offset unit is coupled with the second switching circuit and third switching circuit are controlled by the comparator for selection between the second switching circuit and the offset unit.
Other technical advantages of the present disclosure will be readily apparent to one skilled in the art from the following figures, descriptions, and claims. Various embodiments of the present application obtain only a subset of the advantages set forth. No one advantage is critical to the embodiments. For example, one embodiment of the present application may only provide the advantage of controlling the pixels of a liquid crystal display, while other embodiments may provide several of the specified and apparent advantages.